The organ at First Baptist Church was built as Opus 727 of the E. & G.G. Hook and Hastings Company of Boston, Massachusetts. It is one of the largest, extant Hook and Hastings organs "in their unique full-blown 'American Romantic' style," exceeded only by their Opus 801 at Boston's Cathedral of the Holy Cross.

As indicated on the Opus list for Hook & Hastings Organs, this is a three manual organ (manual compass is 61 notes) with a Pedal compass of 32 notes. There are 56 registers. A listing of specific stops and other specifications can be found at the
Organ Historical Society's Pipe Organ Database.

The organ (and the building in which it resides) was originally built for the Brattle Square Society (a Unitarian Church) of Boston.

Included in Opus 727 are at least eight stops (sets of pipes) from the Brattle Square which was built in 1792 by Samuel Green in England.

From Green's Great Organ of 1792, the Open Diapason, Stopped Diapason, Principal , Twelfth, Fifteenth, Sesquialtera and Mixture pipes are included in Opus 727. Of Green's Swell Organ, the Stopped Diapason was included.

Since being built by E. & G.G. Hook and Hastings in 1873, Opus has undergone numerous alterations. In 1882, when First Baptist Church obtained it, the (tracker-action) console was moved from the side to the front of the organ case. In 1927, William W. Laws converted the organ to electrical action.